Methicillin-resistant Staphylococcus aureus (MRSA) strains contain a SCCmec cassette, which may contain the mecA gene whose gene product confers methicillin resistance. Specific detection of MRSA by the presence of the SCCmec cassette has been used in a number of publications and patents (U.S. Pat. No. 6,156,507, US 2005019893). These various methods all rely on the use of primer set(s) where one (or more) primers target the S. aureus chromosomal DNA (external to SCCmec) and the other one (or more) primers target the SCCmec cassettes near the chromosomal integration site. Due to the presence of multiple SCCmec (sub) types, more than one primer set is needed to detect all MRSAs. SCCmec cassettes are also found in other Staphylococcus species, commonly known as coagulase-negative staphylococci, including S. epidermidis, S. hominis, S. haemolyticus and others. Current methods therefore use primers directed specifically to S. aureus chromosomal DNA (non-cassette regions) to assess the species identification and to avoid confusion between SCCmec cassettes detected in S. aureus and those in other staphylococci.
In theory, this is a clear strategy for MRSA detection, however, in practice this concept produces both false positive and false negative results as has been reported (Drews, et. al., J. Clin. Microbiol. 44:3794, 2006; Desjardins, et. al., J. Clin. Microbiol. 44:1219, 2006, Bishop, et. al. J. Clin. Microbiol. 44:2904, 2006) and there is therefore a need for improved methods with higher sensitivity and specificity.